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von Willebrand factor
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Issue number 4 2012
von Willebrand factor (vWF):
Overview 1
•A multimeric glycoprotein that promotes platelet
adherence and aggregation and is a carrier for FVIII in
plasma
•Protein assembled from identical subunits into linear
strings, referred to as multimers, varying in size from
~2–20 million Da
•Multimers are subjected to physiologic degradation
by the metalloprotein ADAMTS13 (A Disintegrin-like
And Metalloprotease domain [reprolysin type] with
Thrombospondin type I motifs)
•The FVIII-vWF complex circulates as a loosely coiled
protein complex
•When vascular injury occurs:
•vWF (particularly the large multimers) becomes
tethered to the exposed subendothelium (collagen, etc.)
•High fluid shear rates in the microcirculation induce
a conformational change (‘unrolling’ or ‘uncoiling’) in
multimeric vWF
•This then binds to the platelet membrane receptor
glycoprotein (GP) Ib complex, leading to platelet
adherence, activation and aggregation, and facilitating
clotting
•vWF acts as a specific carrier protein for Factor VIII in
the plasma
•Protects Factor VIII from proteolytic degradation
•Factor VIII is released from vWF by the action of
thrombin
•vWF therefore helps target Factor VIII to sites of
vascular injury
•vWF therefore plays a major role in blood coagulation
and vWF deficiency or dysfunction (von Willebrand
disease) leads to a bleeding tendency
Role of von Willebrand
factor in normal hemostasis
Platelet adhesion
Platelet activation
Platelet aggregation
FVIII
Thrombin
GPIIb/IIIa
receptor
Platelet
GPIa
vWF
Platelet
activators
vWF binds to exposed
subendothelial
collagen receptors
at site of injury
and interacts
with platelet GPIb
FVIII
Fibrinogen
Coagulation
cascade
Thrombin
generation
Activated platelets
express
GPIIb/IIIa receptor
GPIIb/IIIa receptor
binds to circulating
fibrinogen and
vWF to cross-link
platelets
vWF = von Willebrand factor; GP = glycoprotein
Fig. 1
Inheritance of
von Willebrand disease
Autosomal dominant:
Type I and Type II vWD
Affected male Unaffected female
Autosomal recessive:
Type III (some Type II) vWD
Carrier male Carrier female
Affected Unaffected Unaffected Affected Affected Unaffected Carrier
child
son
daughter
son
daughter child
child
1:4
chance
Fig. 2
1:4
chance
Carrier
child
1:2
chance
Classification of
von Willebrand disease1
Type
Description
Inheritance Prevalence
Bleeding
propensity
1
Partial quantitative
vWF deficiency
Qualitative vWF
defect
Decreased vWFdependent platelet
adhesion with
selective deficiency
of high-molecularweight multimers
Increased affinity for
platelet GPIb
Decreased vWFdependent platelet
adhesion without
selective deficiency
of high-molecularweight multimers
Markedly decreased
binding affinity for
FVIII
Virtually complete
deficiency of vWF
Defect in platelet
GPIb
AD
Up to 1%
Mild-moderate
AD (R)
Uncommon
Variable – usually
moderate
AD
Uncommon
AD (R)
Uncommon
Variable – usually
moderate
Variable – usually
moderate
AR
Uncommon
Variable – usually
moderate
AR
Rare (1:250,000
to 1:1,000,000)
Very rare; may be
misdiagnosed as
type 2B
High – severe
bleeding
Variable – usually
moderate
2
2A
2B
2M
2N
3
Platelettype
AD
Table 1
AD autosomal dominant; AR autosomal recessive; AD (R) autosomal dominant (may be recessive)
Summary of von Willebrand
disease assays
Test
Function
Assay
von Willebrand factor
ristocetin cofactor assay
(vWF RCo)*
Assesses binding activity of
vWF to platelets
von Willebrand factor
antigen (vWF Ag)**
vWF protein as measured
by protein assays; does not
imply functional ability
Reflection of vWF activity to
antigen concentration and
aids differentiation between
quantitative and qualitative
differences in vWF
Measures the cofactor
function of FVIII in plasma,
and therefore indicates the
ability of vWF to bind and
maintain the level of FVIII in
the circulation
Qualitative assessment of
size of vWF multimers
Ristocetin cofactor activity:
quantitates platelet agglutination after addition of vWF
and ristocetin
Immunoassays such as
ELISA, LIA, RIA
vWF RCo: vWF Ag
Factor VIII (FVIII)
von Willebrand factor
multimers
Ristocetin-induced
platelet aggregation
(RIPA)
Ratio of vWF RCo to vWF Ag
FVIII activity: plasma clotting
test based on PTT assay
using FVIII-deficient
substrate; quantitates activity
Electrophoresis in agarose
gel and visualisation with
monospecific anti-vWF
Measures the ability of vWF
Measures aggregation
to bind to platelets in the
of platelet-rich plasma to
presence of various concent- various concentrations of
rations of ristocetin
ristocetin
RIPA is similar to vWF RCo,
but ristocetin is added
directly to the plasma
without serial dilution
Table 2
* Platelet-based assays preferred
**ABO blood types have a significant effect on plasma vWF (and FVIII) concentrations.
Individuals who have blood type O have concentrations approximately 25% lower
compared to persons who have other ABO blood types.
Laboratory evaluation1
Positive
Initial evaluation
(history and physical
examination)
Laboratory evaluation
Initial hemostasis tests
• CBC and platelet count
• PTT
• PT
• Fibrinogen or TT
(optional)
No further evaluation
Isolated prolonged PTT that
corrects on 1:1 mixing study**
or no abnormalities
1 or more tests
abnormal
IF bleeding history is
strong, consider
performing initial VWD
assays
Other cause identified,
e.g., platelets*, isolated
abnormal PT,
low fibrinogen,
abnormal TT
Possible referral for other
appropriate evaluation
Negative
Initial VWD assays
• vWE:Ag
• vWF:RCo
• FVIII
No test abnormal
Referral for selected
specialized vWD studies
• Repeat initial vWD
assays if necessary
• Ratio of vWF:RCo to
vWF:Ag
• Multimer distribution
• Collagen binding
• RIPA or platelet binding
• FVIII binding
• Platelet vWF studies
• DNA sequencing of
vWF gene
Referral for other
appropriate evaluation
Fig. 3: Laboratory assessment for vWD or other bleeding disorders.
The diagnosis, evaluation and management of von Willebrand disease
(2007). Full report, NIH Publication No. 08-5832, December 2007.
* Isolated decreased platelets may occur in VWD type 2B.
**Correction in the PTT mixing study immediately and after 2-hour incubation remo
ves a factor VIII (FVIII) inhibitor from consideration.
Investigation of other intrinsic factors and lupus anticoagulant also may be indicated.
CBC, complete blood count; PT prothrombin time;
PTT partial thromboplastin time; RIPA, ristocetin-induced platelet
aggregation; TT, thrombin time; vWF:Ag, vWF antigen; vWF:RCo,
vWF ristocetin cofactor activity.
Laboratory classification of
von Willebrand disease
Expected laboratory values in VWD
Normal Type 1 Type 2A Type 2B Type 2M Type 2N Type 3 PLTVWD*
VWF:Ag N
L, ↓ or ↓ or L
↓ or L
↓or L
N or L
absent ↓ or L
↓↓
↓ ↓or
↓ ↓↓
N or ↓
↓↓
↓↓
N or L
absent ↓ ↓
N or ↓
N or ↓
↓↓
N or L
often N
↑↑↑
↑
↓
absent
↑
N
absent
N
1-9 IU/
dL
absent
absent
↑↑↑
↑
↓ or N
↑
↓
FVIII
N
L, ↓ or
↓↓
N or ↓
RIPA
LD-RIPA
PFA100®
CT
BT
Platelet
count
VWF
multimer
pattern
N
absent
N
often N ↓
absent absent
N or ↑ ↑
N
N
N or ↑
N
↑
N
↑
N
N
N
↑↑↑
N
N
N
abnormal abnormal N
N
absent abnormal
VWF:Rco N
often N
↑↑↑
↑
Table 3
The symbols and values represent prototypical cases. In practice, laboratory studies in
certain patients may deviate slightly from these expectations.
L, 30-50 IU/dL; ↓, ↓↓, ↓↓↓, relative decrease; ↑, ↑↑, ↑↑↑, relative increase; BT, bleeding
time; FVIII, factor VIII activity; LD-RIPA, low-dose ristocetin-induced platelet aggregation (concentration of ristocetin ≤ 0.6 mg/mL); N, normal; PFA-100® CT, platelet function
analyzer closure time; RIPA, ristocetin-induced platelet aggregation; VWF, von Willebrand
factor; VWF antigen; VWF:Rco, VWF ristocetin cofactor activity.
*Note: persons who have platelet-type VWD (PLT-VWD) have a defect in their platelet
GP1b. Laboratory test results resemble type 2B VWD, and both have a defect in their
LD-RIPA. In the VWF:platelet binding assay, persons who have type 2B VWD have
abnormally increased platelet binding. Normal persons and those who have PLT-VWD
have no binding of their VWF to normal platelets at low ristocetin concentrations.
Special considerations
for laboratory
diagnosis of vWD
Repeated testing for vWD is sometimes needed to
identify low levels of a
• A number of factors (e.g. stress, surgery, pregnancy,
systemic inflammation, oral contraceptives) can lead to
increased plasma vWF levels
Considerations in sample preparation
•Ensure phlebotomy is as calm as possible to avoid
stress-induced increases in vWF
•Fasting or non-lipidaemic samples should be used;
icteric or haemolysed samples may compromise the
quality of testing results
•Blood should be centrifuged promptly to obtain
plasma and stored at room temperature if tests are to
be performed within 2 hours. Frozen plasma samples
should be thawed at 37°C to avoid cryoprecipitate
•Plasma should be ‘platelet-poor’ or ‘platelet-free’; care
is required during centrifugation; double centrifugation
may be of benefit
The vWF reference standard is critical to the
laboratory diagnosis of vWD
•Where possible, all laboratories should use the same
standard to avoid artifactual discrepancies
•Results of vWF assays can only be reported in
international units (IU) if referenced to the WHO
standard; if a reference plasma pool is used, it is
usually reported as a percentage of normal
Variability
•The variability (CV) of the vWF:RCo assay and of
the vWF:Ag assay is high (≥20–30% and ≥10–20%,
respectively)
•Interlaboratory variability is also considerable
Source of reference
1 The Diagnosis, Evaluation, and Management of von Willebrand Disease (2007).
Full report, NIH Publicaton No: 08-5832, December 2007.
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